The present invention relates to integrated circuits (IC) measurement generally, and particularly to wafer type probes for measurement of electrical characteristics of small planar devices (IC elements) fabricated on semiconductive or dielectric wafers.
Wafer probes provide temporary electrical contacts between test equipment and the very small terminal points (bonding pads) of IC elements on semiconductive wafers. Use of wafer probes permits operation and testing of IC elements prior to separating, bonding and packaging the individual IC elements on the wafer.
A major problem with most existing wafer probes is their inability to permit accurate measurements of the electrical characteristics of the devices when contacting low impedances at high frequencies (e.g., when signal frequencies above approximately 2 GHz are applied). These inaccuracies arise from high frequency characteristics of the probes themselves. At high frequencies, excessive probe inductance or changing probe radiation impedance can greatly reduce the accuracy of the tests made. Although the use of computercorrected measurements may reduce the adverse effect of excess inductance to some extent, the radiation is generally not repeatable and, therefore, not correctable.
These inaccuracies were significantly reduced by the improved wafer probe described and claimed in prior application, Ser. No. 318,084 filed Nov. 4, 1981. The present invention is a further improvement in wafer probe technology alleviating another source of non-repeatable measurement innacuracy which becomes more apparent when inaccuracies due to exccessive probe inductance and changing probe radiation impedance are otherwise reduced.
This additional source of measurement inaccuracy arises in existing wafer probes when some of the energy transmitted or reflected from the device under test excites the transmission line mode resulting from the shield or ground of the probe transmission line acting as one conductor, and the wafer stage, wafer or other nearby conductors acting as the other conductor. This "outer conductor" mode energy typically propagates up the probe board, reflects off the probe mounting block or other discontinuities, and then propagates back down the board. Upon reaching the device under test, some of the "outer conductor" mode energy can couple back into the normal modes, while the remainder is dissipated in the device under test, and/or reflected back up the probe. Thus a resonator results from the energy alternately reflecting from the device under test and from the probe mounting area. While the "outer conductor" mode resonates on the probe board, some of its energy is also radiated.
This radiation from the "outer conductor" mode resonance is particularly undesirable since the radiated energy reflects back from the conductors in the vicinity of the wafer stage and these conductors typically move in relation to the wafer, creating non-reproducible measurements.
What is needed and would be useful, therefore, is a low excess inductance wafer probe capable of making accurate on-wafer measurements by minimizing errors due to excessive probe inductance and changing probe radiation impedance and by minimizing radiation due to transmission line mode resonance along the probe ground.